1. Field of the Invention
The present invention relates to a memory system, and particularly to a memory system using a flash EEPROM non-volatile memory, for example.
2. Related Art
Semiconductor memories are employed for main storages of large computers, personal computers, household electric appliances, cell phones and the like. Flash EEPROM non-volatile memories (hereinafter, “NAND flash memories”) are applied to various memory mediums (such as SD cards, MMCs (Multi Media Cards), MS (Magnetic Stripe) cards, CF (Compact Flash) cards, USB memories, SSDs (Solid-State-Disks)). The NAND flash memories are used as information (images, videos, sound, and games) storage mediums for cell phones, digital cameras, digital video cameras, music devices such as MP3, mobile devices, digital televisions and the like. Further, the NAND flash memories are used as storage mediums instead of HDDs in personal computers (SSDs (Solid State Drives)).
According to the NAND flash memory, a data writing/reading operation is performed per page formed of a plurality of memory cells and the data is erased per block formed of a plurality of the pages.
The NAND flash memory has limited numbers of data readings, writings, and erasures. The data in the NAND flash memory is usually deteriorated by the number of data readings, writings, or erasures of around 105. When the NAND flash memory is used as the SSD, its lifetime will end in a short period of time.
Further, even if data with lower capacity than a block's capacity is written/read more frequently, the data is erased per block. The number of erasures is therefore increased, causing the memory to be deteriorated rapidly. Generally, the low capacity data is handled more often than data with higher capacity than the block's capacity such as image data and music files in a personal computer use environment.
The deterioration of the NAND flash memory may be suppressed by using RAMs including DRAM and FeRAM as a buffer or cache of the NAND flash memory. However, the NAND flash memory for SSD has recently a high capacity. It is thus difficult to obtain RAMs with sufficiently high capacity to suppress the deterioration of the NAND flash memory. This is because a user available area is reduced.
According to the NAND memory, the shorter an erasing time interval becomes, the worse a retention characteristic (data retention characteristic) becomes. Recently, systems for extending the erasing time interval are required.